Sensors are used in a variety of modem devices and transducers. Tactile sensors are useful in a wide variety of applications for robotics and computer hardware. In robotics, tactile sensors provide useful information about the state of contact between a robot hand and an object in prehension. Sensors can indicate the presence or shape of an object, its location in the hand, and the force of contact. However, most robotic sensors are based on a pressure sensor design and can measure only compressive force without regard to shear movement. Shear sensors would be useful, for example, in detecting the movement of a grasped object.
Most controls for computer-related products are pressure-sensitive devices such as keys. Controllers such as the computer "mouse" or the computer "joy-stick" respond to movement in two dimensions but are relatively complex to manufacture and subject to mechanical failure. Accordingly, there exists a need for simple, compact tactile shear sensors for robotic and computer applications. High resolution tactile sensors are useful for accurate control devices such as for high-density, miniature computer products, for highly sensitive robotic skin sensing, or for touch-sensitive virtual reality devices such as control gloves worn by a remote operator or the fingers of a robot used for surgical operation of patients.
Nano-scale wires such as carbon nanotubes with a very small size scale, on the order of 1-100 nanometers in diameter and 0.1-100 .mu.m in length, have received considerable attention in recent years. See Liu et al., Science, Vol. 280, p. 1253 (1998); Ren et al., Science, Vol. 282, p. 1105 (1998); Li et al., Science, Vol. 274, p. 1701 (1996); Frank et al., Science, Vol. 280, p. 1744 (1998); J. Tans et al., Nature, Vol. 36, p. 474 (1997); Fan et al., Science, Vol. 283, p. 512 (1999); Collins et al., Science, Vol. 278, p. 100 (1997); Kong et al., Nature, Vol. 395, p. 878 (1998); and Ebbesen et al., Nature, Vol. 382, p. 54 (1996).
Carbon nanotubes exhibit unique atomic arrangements, nano-scale structures and interesting physical properties such as one-dimensional electrical behavior, quantum conductance, and ballistic transport characteristics. The ballistic transport in carbon nanotubes, as reported by Frank et al, allows the passage of huge electrical currents in electronic circuits, with the magnitude of current density comparable to or better than those in some superconductors. Carbon nanotubes are one of the smallest dimensioned nanowire materials with generally high aspect ratio and small diameter of .about.1 nm in the case of single-wall nanotubes and less than .about.50 nm in the case of multi-wall nanotubes. See Rinzler et al, Appled Physics, Vol. A67, p. 29 (1998); Kiang et al, J. Physical Chem., Vol. 98, p. 6612 (1994), and Kiang et al, Physical Review Letters, Vol. 81, p. 1869 (1998).
High-quality single-walled carbon nanotubes are typically grown as randomly oriented, needle-like or spaghetti-like, tangled nanotubes by laser ablation or arc techniques (a chemical purification process is usually needed for arc-generated carbon nanotubes to remove non-nanotube materials such as graphitic or amorphous phase, catalyst metals, etc). Chemical vapor deposition (CVD) methods such as used by Ren et al., Fan et al., and Li et al tend to produce multiwall nanotubes attached to a substrate, often with a semi-aligned or an aligned, parallel growth perpendicular to the substrate. As described in these articles, catalytic decomposition of hydrocarbon-containing precursors such as ethylene, methane, or benzene produces carbon nanotubes when the reaction parameters such as temperature, time, precursor concentration, flow rate, are optimized. Nucleation layers such as a thin coating of Ni, Co, Fe, etc. are often intentionally added to the substrate surface to nucleate a multiplicity of isolated nanotubes. Carbon nanotubes can also be nucleated and grown on a substrate without using such a metal nucleating layer, e.g., by using a hydrocarbon-containing precursor mixed with a chemical component (such as ferrocene) which contains one or more of these catalytic metal atoms. During the chemical vapor decomposition, these metal atoms serve to nucleate the nanotubes on the substrate surface. See Cheng et al., Chem. Physics letters, Vol. 289, p. 602 (1998).
The as-grown single-wall nanotubes (SWNT) such as commonly synthesized by laser ablation or arc method, have a spaghetti-like configuration and often are tangled with each other. The multi-wall nanotubes (MWNT), such as commonly made by chemical vapor deposition, are easier to prepare in an aligned and parallel configuration. However, these as-grown nanotubes such as reported by Ren et al. and Li, et al. differ in height or length. Applicants have discovered a high-resolution tactile sensor may be fabricated with nanowires vertically attached to a sensor substrate such that, upon tactile contact, the nanowires make physical and electrical contacts between them such that presence of tactile shear or compression contact can be determined by electrical interrogation. For reliable tactile sensors as disclosed in this invention, the nanowires should be substantially vertically aligned and of equal length, such that prior methods of making SWNT and MWNT are generally unsuitable for the inventive high-resolution tactile sensors.